This PR expands `mvcgen using invariants | $n => $t` to `mvcgen; case
inv<$n> => exact $t` to avoid MVar instantiation mishaps observable in
the test case for #9581.
Closes#9581.
This PR makes `mvcgen` aggressively eta-expand before trying to apply a
spec. This ensures that `mspec` will be able to frame hypotheses
involving uninstantiated loop invariants in goals for the inductive step
of a loop instead of losing them in a destructive world update.
This PR makes `mvcgen` produce deterministic case labels for the
generated VCs. Invariants will be named `inv<n>` and every other VC will
be named `vc<n>.*`, where the `*` part serves as a loose indication of
provenance.
